There are various starting materials for the preparation of isobutylene including the residue (hereinafter referred to as "spent BB") left after extraction of butadiene from a BB fraction (butene-butadiene fraction) obtained as a by-product at cracking of naptha. Also, there is an isobutylene-containing hydrocarbon mixture such as a decomposition gas formed by fluidized catalytic decomposition of light oil (kerosine). However, in each of these starting materials, normal butene isomers and butanes having a boiling point close to that of isobutylene are also present with the isobutylene. Thus from a cost of effectiveness viewpoint, separation of isobutylene by customary distillation is difficult. Thus in the industrial process customarily used in this art, the characteristic property of isobutylene, that is a higher reactivity as compared with the reactivity of other butenes, is utilized, and an isobutylene-containing hydrocarbon mixture is contacted with, for example, a 50 to 70% by weight aqueous solution of sulfuric acid to selectively extract isobutylene.
As is well recognized in the art, this process is defective in that special materials are needed to fabricate the equipment used because sulfuric acid is employed. Further, undesirable oligomers of isobutylene, such as a dimer, are formed in large quantities. In addition, the purity of isobutylene obtained according to this process is 97 to 98%, and in order to increase the purity to 99.9% or more, additional operations such as separation using a molecular sieve and another sulfuric acid absorption should be carried out, resulting in increased manufacturing costs.
Considerable attention has been given in this process in order to eliminate these defects. The main object of these studies is to isolate a reaction solvent or catalyst that provides a higher reactivity with isobutylene and a lower reactivity with other butenes, in other words, a reaction solvent or catalyst having a high selectivity to isobutylene and providing a high productivity so as to fully utilize the difference of isobutylene over other butenes in respect to their relative reactivities.
In order to improve the purity of the isobutylene final product, it is one of the important requirements that the selectivity of the reaction system should be high. Furthermore, in order to obtain high purity isobutylene it is important to use a heterogeneous catalyst, which allows easy separation of the catalyst from the reaction product, as the catalyst for reaction of isobutylene. More specifically, an unreacted hydrocarbon mixture is contained more or less in a liquid reaction mixture obtained by the reaction of isobutylene, and if this unreacted hydrocarbon mixture is not separated from the reaction product of isobutylene, it is incorporated into isobutylene when isobutylene is obtained by the decomposition of the reaction product of isobutylene, resulting in a reduction of the purity. When a catalyst is present at the step of separating the reaction product of isobutylene from this unreacted hydrocarbon mixture, the reaction product of isobutylene is decomposed, and even if high productivity and high yield are both attained in the reaction, both the productivity and yield are reduced by this decomposition and good results are not obtained. Accordingly, a heterogeneous catalyst such as an ion exchange resin is preferred as the catalyst for reaction of isobutylene.
A process for the preparation of isobutylene, based on the above-mentioned concept, is disclosed in, for example, U.S. Pat. No. 3,026,362. This process comprises separating isobutylene in the form of a carboxylic acid ester from a hydrocarbon mixture in the presence of a saturated aliphatic carboxylic acid having 1 to 4 carbon atoms by using an acidic ion exchange resin having a macro-reticular structure as the catalyst, and decomposing the ester by using as the catalyst the same acidic ion exchange resin. In this method, the carboxylic acid ester is formed in a high yield at a high productivity, but the difference of isobutylene as opposed to normal butenes in respect to the reactivity is not sufficient. For example, when isobutylene in spent BB is reacted with acetic acid and is converted to tertiary butyl acetate according to this process, secondary butyl acetate is formed as a by-product in an amount of about 1%. Accordingly, if the reaction product is decomposed, the purity of the resulting isobutylene is about 99%.
Another process has also been proposed in which isobutylene is hydrated in a hydrocarbon mixture by using a solvent and a catalyst and separating isobutylene in the form of tertiary butyl alcohol. Whatever solvent and catalyst may be used in this process, however, the tertiary butyl alcohol so formed contains a considerable amount of secondary butyl alcohol which is undesirable as it is not suitable for production of high purity isobutylene. Accordingly, in order to obtain high purity isobutylene, there should be adopted a method for dehydration of tertiary butyl alcohol, as disclosed in, for example, Soviet Union Pat. No. 202,881, in which tertiary butyl alcohol is separated from secondary butyl alcohol by fine distillation utilizing a slight difference of the volatility between tertiary butyl alcohol and secondary butyl alcohol. Thereafter, the thus separated tertiary butyl alcohol is dehydrated and decomposed.
The present applicants have studied such reactions with a view to developing a cost effective process capable of producing high purity isobutylene having a purity of at least 99.9% with economical advantages, and as the result, we have found that high purity isobutylene can be prepared when a reaction gas obtained by treating a mixture containing tertiary butyl alcohol and a tertiary butyl ester of an aliphatic carboxylic acid having 1 to 6 carbon atoms with an acidic ion exchanger is subjected to customary distillation separation.
The mixture of tertiary butyl alcohol and tertiary butyl ester as described above is obtained according to a method disclosed in, for example, U.S. Pat. No. 4,011,272, the disclosure of which is hereby incorporated by reference. More specifically, an isobutylene-containing hydrocarbon mixture and an aqueous solution of an aliphatic carboxylic acid having 1 to 6 carbon atoms are reacted in the presence of an acidic ion exchanger. The unreacted hydrocarbons are separated from the reaction mixture by distillation to form a residual reaction mixture liquid (I). Then, a mixture (II) containing tertiary butyl alcohol and a tertiary butyl ester of the carboxylic acid is obtained together with a portion of water from liquid (I) by conventional distillation.
According to this procedure, we obtained mixture (II), and the content of secondary butyl alcohol, which is a by-product, and secondary butyl ester of the carboxylic acid, which is also a by-product, in the mixture (II) were determined with improved analysis accuracy. It was found that the secondary butyl alcohol and the secondary butyl ester of the carboxylic acid were together present in a total amount of 0.3 mol%. This means that even though the hydration conditions disclosed in U.S. Pat. No. 4,011,272 provides a good selectivity with respect to isobutylene, the purity of isobutylene that can be expected according to this process is about 99.7%. In order to further improve the purity, one should use a special method of removal of secondary butyl alcohol and secondary butyl ester of the carboxylic acid by fine distillation.